Method of forming thin film resistors by cathodic sputtering



J. H. HALL Nov. 11, 1969 METHOD OF FORMING THIN FILM RESISTORS BY CATHODIC SPUTTERING Filed June 7. 1966 'FIGZA.

INVENTOR JOHN H. HALL Y B Lu q ATTORNEY FIGZE.

United States Patent C) 3,477,935 METHOD OF FORMING THIN FILM RESISTORS BY CATHODIC SPUTTERING John H. Hall, Santa Clara, Calif., assignor to Union Carbide Corporation, a corporation of New York Filed June 7, 1966, Ser. No. 555,771 Int. Cl. C23c 15/00 US. Cl. 204-192 Claims ABSTRACT OF THE DISCLOSURE A method of forming thin film resistors by cathodic sputtering of a silicon target wound thereabout with wires of a nickel-chromium alloy.

other such materials to form thin film resistor structures,

but these processes have been found to suffer from various difiiculties such as economic producibility compatibility with other process steps in device fabrication, etc. The deposited resistors additionally have been found acking in reliability, stability and ease of contacting.

More specifically the characteristics needed in a thin film resistor are: ability to withstand exposure to oxidizing atmospheres at temperatures in excess of about 450 C. for periods of time of up to 15 minutes; resistance to scratching by metals or silicon dust; should be. corrosion resistant and nonreactive with silicon oxide and contacting materials, but should form good contacts with such materials; should have superior electrical performance properties such as high ohms per square, low temperature coefficient, good load life stability, and capability of operating at high power densities; and should be reproducible with high precision and preferably formable with photoresist techniques and otherwise compatible with current methods of device fabrication.

Nichrome, tantalum and other similar thin film resistor materials are oxidizable. Films of these materials are unstable at high temperatures due to crystal grain growth and grain boundary diffusion of gases and metals.

Nichrome films on silicon oxide surfaces can react with the SiO air, or aluminum from the interconnection pattern evaporation. These reactions can cause the resistor material to change value or to fail completely. Tantalum films suffer from similar disadvantages; for example, it has been found that gold and aluminum contacting materials diffuse readily in sputtered tantalum films.

It is the object of this invention, therefore, to provide a resistor composition and structure and method for its application which overcome the above noted deficiencies.

It is a further object of this invention to provide resistor structures which have characteristics far superior to previous devices and which are specifically vary stable at high temperatures, and over long load life periods and which can operate at high power levels.

It is another object of the invention to provide a method for applying such thin film resistors, which method has high yields in manufacturing and is compatible with other process steps involved in device fabrication and which is economical, and which gives highly reproducible results.

These and other advantages of the present invention will 3,477,935 Patented Nov. 11, 1969 we 1C be apparent from the following description, the appended claims and the attached drawings.

In accordance with these objects a stable thin film resistor is provided composed of nickel, chromium and silicon simultaneously co-deposited on a substrate.

In preferred forms of the invention the nickel and chromium are present in the relative proportions of about percent by weight nickel and about 20 percent by weight chromium, the nickel and chromium together amounting to from about percent by weight to about 5 percent by Weight of the resistor composition, with the balance substantially all silicon; and in another embodiment, nickel is present as about 75 percent by weight with about 20 percent by weight chromium and about 2.5. percent by weight each of aluminum and copper, the nickel, chromium, aluminum and copper amounting to from about 95 percent by weight to about 50 percent by weight of the resistor composition, with the balance substantially all silicon. J

The method of applying the nickel, chromium and silicon resistor comprises simultaneously sputtering nickel, chromium and silicon onto a substrate to form a film thereon composed of nickel, chromium and silicon. The process preferably employs ion sputtering as more fully set forth hereinafter.

In the drawings:

FIG. 1 is a schematic front elevational view of an apparatus suitable for use in producing the film resistors of this invention.

FIGS. 2A through 2E are plan and sectional views of a thin film resistor structure, greatly exaggerated, in accor'dance with the present invention in various stages of fabrication.

With reference more particularly to the drawings, FIG. 1 shows apparatus suitable for depositing a film resistor of this invention by ion sputtering. Shown in FIG. 1 is a vacuum chamber or belljar 10 in which is disposed an anode 11 on a support 12 also providing an electrical connection. Filament 13 located in passageway 14 generates electrons which are accelerated through the duct 15 and into the chamber towards the anode 11 by the electrical potential between the cathode filament 13 and the anode 11. The ends of the filament 13 are connected to electrical leads 16 leading to a power supply 17 with one lead grounded. Suitable operating voltages for the filament are between 6 and volts AC or DC. The anode is maintained at a positive voltage between 20 and 100 volts, typically 55 volts.

A target body 18 is supported on an arm 19 connected to a source of negative potential between 50 and -5000 volts, and typically -1000 volts. A conduit 20 provided with a valve (not shown) allows for the introduction of gases, for example argon, which is ionized by collisions with the stream of electrons flowing from the filament to the anode. Argon ions from this plasma are accelerated toward the target body 18 with a velocity sufiicient to cause vaporization and removal of target body material from the target. The vaporized target body material condenses on the surfaces of a substrate 21, shown having a mask 22 with an opening 22a defining the geometry of a desired film resistor structure.

The substrate 21 is supported on an arm 23 and is maintained at a desired temperature, generally about (3., by a heater (not shown).

The chamber is evacuated by a conduit leading from the belljar to suitable pumps (not shown).

The target body is the source of the constituents of the resistor compositions of this invention. This target body may advantageously be composed of a silicon body 24 wrapped with numerous windings of wire 25 having a nickel-chromium, or nickel chromium-aluminum-copper composition. Depending on the closeness of the Wire wrappings, and hence the relative exposed areas of silicon and wire, the composition of the material sputtered and also the composition of the material condensed on the substrate can be varied. A silicon target body having a large number of closely spaced wire wrappings will yield, on exposure to the plasma, a stream of target body material higher in nickel and chromium content; and conversely, a silicon target body having a lesser number of windings of wire and spaced further apart will give a stream of sputtered material higher in silicon content.

The pressure of inert gas in the chamber may be varied from about 05x10" torr to about 1x10" torr, depending on the desired rate of material removal from the target body. Increasing the inert gas pressure and thereby reducing the vacuum within the chamber increases the rate of sputtering. The minimum pressure represents the lower range of deposition rates. The target voltage and electron current also determine the sputtering rate. These parameters can be adjusted to give a desired sputtering rate in the chamber. Thickness of the deposited material is then a function of the time of exposure to the sputtered stream.

FIG. 2A shows a plan view and FIG. 2B shows a sc tional view of a portion of a substrate 26 upon which an elongated thin film resistor structure 27 has been deposited. The surface of the substrate could have been covered with a film of SiO but is shown bare for simplicity. This surface was covered with a layer of photoresist during the sputtering operation, except for an opening in the photoresist having the shape and dimensions of the desired resistor structure. The photoresist was removed after sputtering, leaving the resistor structure shown.

FIGS. 2C through 2E show a method for attaching electrical contacts to the extremities of the resistor structure, as follows: a molybdenum coating 28 is applied over the surface of the substrate 26 covering the resistor 27, as by sputtering of molybdenum in an apparatus similar to that previously described; a second coating 29 of gold is then applied over the molybdenum coating 28. By masking and etching techniques, the gold and then the molybdenum coatings are removed from all surfaces of the substrate and resistor structure except for electrical contact areas 30 and 31 located at either end of the resistor 27, and extensions 32 and 33 of these contacts 30 and 31. These extensions 32 and 33 may serve as interconnections to the electrodes of other active or passive devices located in or on other portions of the substrate 26, not shown here, and in a manner known to the art. The molybdenum-gold contacts and interconnections may be manufactured simultaneously over the whole of a suitably masked substrate having a variety of devices formed therein, including the resistor structure of this invention. Aluminum or other contact material could be used instead of the molybdenumgold described above, all of which are compatible with current integrated circuit manufacturing procedures.

The resistors manufactured by the process of this invention are believed to have a very fine polycrystalline structure with distinct grain boundaries characteristic of sputtered deposits. The composition of this polycrystalline compound is com-posed of nickel and chromium with the balance substantially all silicon. The composition of the deposited compound can be varied by varying the nature of the target body for it has been found that composition of the deposited resistor corresponds to the composition of the target body, as represented by the relative areas of silicon and wire windings.

The target body may be composed of a flat stab of high purity poly-crystal silicon. The Wire may be a Nichrome wire having a composition of about 80 percent by weight nickel and 20 percent by weight chromium, or other similar material such as Evan-ohm wire, the trade-name of a very low temperature coefilcient of resistance wire having a composition of about 75 percent by weight nickel, about 20 percent by weight chromium, about 2.5 percent by weight aluminum and about 2.5 percent by weight copper. A resistor deposited as a result of sputtering a target body of silicon wrapped with Nichrome wire will have a composition of nickel and chromium in about the same proportions as the nickel and chromium contents of the Nichrome wire. Similarly, if the silicon target body is wrapped with Evan-ohm wire, then the resistor will have a nickelchromium-aluminum-copper content in the same proportions as in the Evan-ohm wire. The amount of silicon associated with these materials in the deposited compound will depend on the amount and spacing of the wire on the target body, i.e., the amount of silicon surface available for sputtering.

The windings of wire on the silicon target body may vary from about 8 turns per inch of 20 mil wire to about 70 turns per inch of the same gauge wire across the length and width of the silicon to produce silicon-nickel-chromiurn compounds having the desired silicon content of from about 5 percent by weight to about 50 percent by weight silicon. Other sizes of wire will call for different arrangements of windings.

In a preferred practice of the invention, the windings of wire are found as follows: a first layer of wire is wrapped around a silicon slab, e.g., 4 x 4 inches in size, at about 70 turns per inch spacing; a second or outer layer of wire is then wrapped at right angles to the first layer and at a spacing of about 8 turns per inch. It has been found that the second or outer layer of wire wrappings contributes a greater amount of sputtered material and hence need not be as tightly spaced as the first or inner layer.

Other compositions of nickel and chromium than those given here may also be used to produce nickel-chromiumsilicon deposited films. Additionally other arraigements of target bodies may be used. It is only necessary that the indicated materials be sputtered simultaneously to produce the nickel-chromium-silicon compounds of this invention.

The sputtered resistor films of this invention have characteristics superior to those of most prior art devices. The sheet resistivity of the deposited films can be varied from about 200 ohms per square to 10,000 ohms per square at a film thickness of about 200 A. depending on the silicon content of the deposited nickel-chromium-silicon compound. Compounds containing silicon in higher amounts in the 5 to 50 percent by weight silicon range will have higher sheet resistivities; and lower contents of silicon in the deposited compound will give lower sheet resistivies. Broadly stated, resistor compounds may be produced having resistivities from about ohms per square to about 225,000 ohms per square. A 300 A. thick film having a sheet resistivity of about 300 ohms per square provides an effective resistor structure for many applications in integrated circuits. Resistors having resistivites of 500 and 1,000 ohms per square are also readly obtainable and useful in present day applications.

The resistor compounds of this invention additionally have exceptional stability and resistance to conditions and environments often found destructive of prior art devices. The nickel-chromiumsilicon resistor compounds of this invention are stable at temperatures in excess of 500 C. and in oxidizing atmospheres. It is believed that the silicon content of these nickel-chromium-silicon compounds provides protection against oxidative destruction of the nickel and chromium constituents. Sputtered as well as evaporated unprotected Nichrome resistors, for example, do not have such resistance to high temperatures or oxidizing atmospheres. Since further processing of the wafer containing the deposited resistor structures may involve exposure to temperatures up to 450 C. as well as oxidizing atmospheres, the resistance of the structures of this invention to such conditions is particularly important.

The nickel-chromium-silicon resistors of this invention are stable within 0.4 percent of their resistor values after 1000 hours in life tests at 200 C., 2 milli-amps and 100 volts. The resistors can be made with resistivities of from 200 to 10,000 ohms per square with temperature coefficients in the range of only +25 to -300 p.p.m./ C. Compounds having higher silicon contents will have more negative temperature coefficients of resistance as is characteristic of polycrystalline silicon films, however, high resistivity resistors can be produced with reasonably low temperature coefiicients.

These resistors also have very good power dissipation capabilities. Currents of milliamps can be handled by a 1 mil wide resistor at power dissipation levels of 100,000 watts per square inch.

The resistor compositions of this invention are resistant to the acids H 80 HNO HCl but can be easily etched in a HF-HNO acid solution.

The following example illustrates a preferred manner of manufacturing these resistors using the apparatus shown in FIG 1. The target is formed by wrapping 20 mil Evanohm wire vertically around a 4 x 4 inch silicon slab at a spacing of 70 strands per inch. A second layer of the same wire is wound horizontally around the body on approximately A; inch centers. The target is placed in the vacuum chamber along with a masked, silicon dioxide covered substrate. The target is sputtered for a short time to stabilize conditions (while the substrate is shielded).

The substrate is then coated at a rate of about 2 A. per second. The substrate is held at a distance of about 2.5 inches from the target. This deposition rate and spacing allows for the introduction of oxygen into the compound which will prevent precipitous grain growth at high temperatures. The resistor is deposited to thickness of about 200 A. giving a resistivity of 300 ohms per square, and then provided with aluminum contacts. An alloy cycle is then performed on the resistor to improve the contact between the aluminum and the resistor surface. This cycle can consist of a heating at 500 C. for about minutes in a nitrogen atmosphere to prevent oxidation of the aluminium. This cycle serves to reduce any contact resistance and also to stabilize any changes which might tend to occur, for example due to grain growth, in the resistors and thus avoid the occurrence of such changes when the device is put into use.

As set forth in the example, oxygen or other gases, such as nitrogen, can be injected into the deposited resistor to increase its bulk resistance.

The Evan-ohm wire wound target is preferred for some applications because the resulting aluminum-containing compound is more resistant to aluminum contact materials, which tend to be soluble in the nickel-chromiumsilicon resistor compounds produced using Nichrome wound targets. There is no such difficulty when molybdenum-gold contacts are utilized.

An important feature of the process described herein is that the superior resistor structures described herein can be reproduced consistently and with great precision without requiring any excessively stringent control or operating procedures.

What is claimed is:

1. A method of forming a stable thin film resistor comprising removing material from a target body comprising nickel, chromium and silicon by ion sputtering, said target body comprising a silicon body having wound thereabout wires comprising nickel and chromium, exposing a substrate to a stream of said removed material, and forming on said substrate a film comprising nickel, chromium and silicon.

2. The method as in claim 1 in which plasma ions are produced by passing a stream of inert gas through a high velocity flow of electrons between a cathode and an anode in an evacuated chamber having a pressure of about 1 10 torr of inert gas, and in which the removal of material from the target body by the plasma ions is produced by maintaining said target body at a negative potential of about 1000 volts.

3. The method as in claim 2 in which the inert gas is argon.

4. The method as in claim 1 in which the target body is a silicon body having wound thereabout wires composed of nickel and chromium in the relative amounts by weight of about 80 percent nickel and about 20 percent chromium.

5. The method as in claim 1 in which the target body is a silicon body having wound thereabout wires composed of nickel, chromium, aluminum, and copper in the relative amounts by Weight of about percent nickel, about 20 percent chromium, about 2.5 percent aluminum, and about 2.5 percent copper.

References Cited UNITED STATES PATENTS 2,189,580 2/ 1940 Hewlett 204-192 2,808,351 10/1957 Colbert et al 338308 2,953,484 10/ 1960 Tellkamp 338-308 3,218,194 11/1965 Maissel 204-492 3,381,255 4/1968 Youmans 338-308 3,325,258 6/1967 Fottler et al. 204192 2,992,918 7/1961 Edwin et al 75171 2,638,425 5/1953 Allen 75171 1,769,229 7/ 1930 Mandell 75-171 FOREIGN PATENTS 712,576 6/1965 Canada.

JOHN H. MACK, Primary Examiner SIDNEY S. KANTER, Assistant Examiner US. Cl. X.R. 

